Power brake boosters

No disc brake conversion is complete without addressing stopping capability

Feature Article from Hemmings Muscle Machines

We have featured numerous articles in the past involving disc brake conversions, both front and rear. HMM#62, November 2008, for instance, featured a tech story by Don Caithness giving you installation instructions for adding a power brake booster and master cylinder on a 1967 Dodge Dart. There are many ways to achieve this extra stopping power--choosing between used parts and new aftermarket add-on kits is only the beginning.

Article continues after advertisement

Whether you have decided to upgrade your brake system to front discs or complete four-wheel discs, it is important to remember that your existing non-power master cylinder and distribution block cannot properly handle the transition. You will be defeating the purpose of converting to this more efficient braking system if you do not also update the rest of the components to match the disc brake's requirements. With this in mind, our column this month will feature several new aftermarket suppliers that offer quality power brake boosters and/or booster/master cylinder combination units.
Drum-type brakes with a single reservoir master cylinder only require 400 to 500 p.s.i. of force to open the wheel cylinders and expand the shoes against the drum. This can be easily accomplished with a basic single reservoir master cylinder along with a driver's foot pressure. The brake pedal also figures into this equation, because it is designed to multiply the force of your foot by up to seven times.
Later dual reservoir master cylinder systems required increased brake force of an additional 300 to 400 p.s.i., which could be compensated for by changing the bore diameter of the master cylinder. Drum brake master cylinders also send about 60 percent of their hydraulic force to the front brakes and 40 percent to the rear. The system is balanced by the size of the ports and/or a junction block, which ensures that each wheel receives equal pressure at the same time, no matter how far it is from the master cylinder.
Disc brake systems are quite different from drums in that they need at least 1,000 p.s.i. of pressure; additionally, the front-to-rear brake force ratio varies. Although early disc brake systems often operated on a 70 percent front and 30 percent rear distribution, newer disc brake systems can be as high as 75 to 80 percent front brake and as little as 20 to 25 percent rear. This is why it is important to not only use a booster and master cylinder package that can handle at least 1,000 to 1,300 p.s.i. of pressure, but also to use a proportioning valve that will split the hydraulic pressure properly between the front and rear.
In order to achieve these higher pressures, power brake boosters were developed. The brake booster uses stored engine vacuum to add anywhere between 350 and 1,400 additional pounds of pressure to your braking system. The amount of the additional pressure depends on the size of the booster you use and whether it is a single or dual-diaphragm booster unit. Generally, the bigger the diaphragm is, the more brake force can be achieved. In vehicles where space limits the booster canister size, such as in Mustangs or many big-block engine configurations, a dual-diaphragm unit can provide additional pressure without taking up the extra space.
As a general rule of thumb, according to the Classic Performance Products brake pressure chart, available on the company's Web site, an 8-inch dual-diaphragm booster or a 9-inch single diaphragm booster will both effectively double the pressure created by an unassisted dual-master cylinder assembly.
Whenever possible and where space allows, it is recommended that you use the largest single-diaphragm unit, as these perform better at lower vacuum pressures. Many pickup trucks use an 11-inch booster canister to provide proper braking pressure.
Several brake companies offer separate boosters and master cylinders to tailor-fit the combination to each vehicle's braking requirements, while others offer complete combination units of master and booster in various diameters and configurations. Listed here are just a few of these suppliers.
MBM Brakes sells 16 different combination units for 1964-'72 Chevrolet cars. With booster canister sizes ranging from 7-inch to 11-inch, MBM also has both a 7-inch and 8-inch dual-diaphragm booster setup available. Its 9-inch "Slimline" offering fits 1955-'70 full-size Chevrolets, and also fits many big-block applications where firewall clearance is an issue.
Additionally, MBM has 37 different add-on power-assist assemblies providing coverage for early Fords from 1933 and up and classic Chevrolets from 1937. Truck booster and master cylinder-combination units are available to mount under the floorboards and come with a new brake pedal. These units can be used in street rods, 1947 and newer Chevy pickups and 1953-'56 Ford trucks. Jeep and Corvette applications are also listed in the MBM catalog.
Classic Performance Products (CPP) specializes in classic Chevrolet system upgrade conversions and has many products available for Sixties and Seventies Chevrolet cars, as well as 1947-'72 Chevy pickups and older Ford pickups. Although most of its disc-brake conversion kits contain a new booster and master cylinder, CPP also offers these units separately to convert your Chevrolet or to build your street rod project. Boosters are available in 7- through 9-inch diameters with single- and dual-diaphragm units listed. Eleven different master cylinders are also available, with prices starting at $59 and ranging up to $199 for a chrome-plated unit.
CPP has recently introduced a complete line of Hydro Boost-type brake units. These boosters use hydraulic pressure, usually from the power steering pump, rather than engine vacuum to multiply your braking force. Hydro Boost units can supply up to 1,300 p.s.i. of pressure and are not reliant on engine performance to create enough vacuum. A new proportioning valve is included with each conversion package. Nine new units are currently available for 1955-'88 Chevrolet vehicles and retail for between $695 and $795. Chrome-plated Hydro Boost units sell for $950 to $1,000. CPP also offers two universal Hydro Boost units for street rod applications, which retail for under $495.
Inline Tube has several power brake conversion options to offer. You can purchase each item separately to design your own conversion or purchase one of the ready-made changeovers. GM A-body and X-body master cylinders are available for less than $100. Boosters are available in 8-inch dual-diaphragm, as well as 9-inch and 11-inch single-diaphragm units. A power-assist booster by itself retails for between $139 and $190. Inline's combination units are available for $275, or you can add the correct proportioning valve for an additional $25 to $50.
As we mentioned in our earlier disc-brake conversion articles, Inline Tube has an extensive inventory of replacement proportioning valves for many applications, and can supply you with all the necessary steel brake lines to complete the installation and have it look as professional as a factory installed unit. Inline's brake coverage includes Mopar A-, B- and E-bodies, as well.
Stainless Steel Brake Corporation has several power-assist conversion units offered in its current catalog. Two units are tailored to fit 1964-'72 GM A-body and F-body muscle cars. A 7-inch dual-diaphragm and a 9-inch single-canister unit are both available and each comes with an adjustable proportioning valve that allows you to dial in the brake bias. The 9-inch unit sells for $253, while the 7-inch dual retails for $268 (or $345 for either unit in chrome). Three more are designed for tight clearances in early Mustangs: a 7-inch unit for 1964-'66 ($269.95), another 7-inch unit for 1967-'69 ($339.95) and a 9-inch booster/master cylinder combination unit for 1967-'69 ($435). SSBC sells many other brake components to help you complete any performance changeover. Residual valves, remote-mount reservoirs, and stainless parking brake cables are all listed in its extensive parts catalog.
An important accessory that you should plan to get along with your power brake combination assembly is the All-In-One Brake System Diagnostic Kit offered by Master Power Brakes. This contains the depth gauge necessary to help you determine which pushrod is needed for your assembly, and it will save you time during installation by eliminating the trial and error of installing and then removing the booster again to install another pushrod. Also contained in the kit are a vacuum gauge, syringe-type brake bleeder and a hydraulic pressure tester. Bleeder fittings and adapters are also included.
Master Power Brakes has many choices for replacement booster and master cylinder combination units, too. Seven different units are offered for a 1969 Camaro, ranging in price from $95 to $370. Nine- and 11-inch singles, as well as 7-, 8- and 9-inch dual-diaphragm boosters, are all available with master cylinder attached.
MP Brakes has extensive coverage for other models; for example, 8-inch dual-diaphragm units are offered for B-body Mopars for $395 and six different 9-inch single-diaphragm configuration units for early Mustangs. The 8-inch dual unit supplies an additional 900 p.s.i. of pressure while the 9-inch single will add about 800 extra p.s.i. Nine-inch dual boosters are also offered for Corvette applications, and 10 different street rod combinations are also listed in their catalog. Additional coverage for non-Chevy applications includes 1962-'78 Chrysler, 1952-'72 Mercury and 1957-'72 Fords.
Mopar enthusiasts can find numerous factory duplicate power booster units from Jim's Auto Parts.
Jim's offers 11 units with coverage on most A-, B- and E-body muscle cars from the mid-Sixties to the early Seventies. Pricing ranges from $189 to $400. Jim's also offers an 8-inch dual-diaphragm add-on unit for Hemi engines for $300.
Unfortunately, not much is available yet for you AMC enthusiasts, though several AMC parts supply houses, such as American Parts Depot, market remanufactured original boosters from 1967-'74 mid-size AMC models that you can install with the proper master cylinder to replace your existing non-power system. However, these components, plus a proportioning valve, would have to be pieced together separately.
Each of the suppliers listed in this article has a very knowledgeable technical staff that can address any fitment or installation questions you might have when attempting this conversion. And they can be a great help in determining exactly how much braking force you will need, based on your vehicle weight, type of driving you are doing and types of front and rear brakes you have on your car.
The prices for all these power brake booster units are very reasonable and will the added stopping power will make you feel better after you have installed the unit by itself or in conjunction with another front, rear or both disc brake conversion.
SourcesMaster Power Brakes
888-351-8785www.mpbrakes.comMBM Brakes
800-231-4125www.mbmbrakeboosters.comClassic Performance Products
714-522-2000www.classicperform.comStainless Steel Brakes
800-448-7722www.ssbrakes.comInline Tube
586-532-1338www.inlinetube.comJim's Auto Parts
603-898-0535www.jimsautoparts.comAmerican Parts Depot
937-678-7249www.americanpartsdepot.com

This article originally appeared in the December, 2008 issue of Hemmings Muscle Machines.